1. Field of the Invention
The present invention relates to a battery power management circuit. More particularly, the present invention provides a circuit topology that needs only a single path to both charge and discharge batteries, and an intelligent switching mechanism to switch between batteries in a multiple battery system. The present invention has general utility wherever battery charging circuits are employed, and where multiple batteries are used; and specific utility in portable devices, for example, portable computers and the like.
2. Description of Related Art
Numerous battery charging topologies exist in the art. For portable device applications, it is desirable to have charger circuits that both control the battery as a power source and permit charging of the battery when an external power source is available. It is also desirable to reduce the component count, so that the charging/discharging circuit can be implemented with relative ease.
Rechargeable batteries, i.e., secondary batteries, have been widely used to provide electrical power for driving battery powered electronic appliances such as, by way of example, portable radio cassette players, portable computers, camcorders, cellular telephones and other devices. Alkaline batteries such as nickel cadmium (Ni--Cd) or nickel metal hydride (Ni--MH) batteries have been generally used as the secondary battery. Recently, lithium ion (Li-ion) batteries with an organic electrolytic cell have gained popularity in high-end portable electronic devices because they exhibit high energy density, low temperature characteristics, and stable storage capability.
Rechargeable batteries require an electronic charger for recharging depleted batteries. A charger should include an internal charger circuit incorporated into the battery powered appliance. A charger will begin charging the battery whenever the device is powered by alternating current (i.e., AC) power. External battery chargers accepting one or more batteries to be charged, are equipped with an independent power supply and connectors.
Although rechargeable batteries have various types of battery chemistry, battery pack voltage, and battery pack capacity, there have been few methods of charging the batteries adopted in battery chargers. Generally, the charging method is either a constant voltage charging process or a constant current charging process. Constant voltage charging applies a constant voltage that is higher in amplitude than the nominal voltage of the battery across the terminals of a battery. Constant voltage charging process is typically used for charging a backup battery where frequent charging and discharging is not occurring. The charging voltage is continuously applied to the battery. On the other hand, the constant current charging process applies a constant current to the battery irrespective of any increase in the voltage across the terminals occurring as the charging progresses. Constant current charging is useful for rapidly charging a battery. Constant current charging however, requires a time limit in order to avoid damage of the battery due to overcharging.
U.S. Pat. No. 5,898,234 provides a power supply circuit that uses a single switch to control both charging and discharging. The circuit is composed of a switch for enabling and disabling a charge current and a charge control unit for controlling the switch in accordance with the charge current, the switch being provided in a path shared by the charge current and a discharge current and including a power feed status detection unit for detecting a normal supply voltage of an external power supply source, an absence of a supply of a voltage from the external power supply source, and a drop in a voltage supplied by the external power supply source, and the charge control unit controlling charging of the electric battery by controlling the switch while the power feed status detection unit detects the normal supply voltage of the external power supply source, closing the switch when the power feed status detection unit detects an absence of a supply of a voltage from the external power supply source or a drop in a voltage supplied by the external power supply source, so as to form a discharge path connecting the electric battery and a load, thus supplying power from the electric battery to the load via the switch. Significantly, this patent discloses a current sensing circuit that monitors the discharge current from the battery. This circuit includes a sensing portion (via a sense resistor), an A/D converter and a processor. The sensed signal is digitized, and the processor controls the gate of the switch based on this value. One disadvantage of this approach is that digital circuits are inherently slower than analog circuits. When an external AC adapter is removed from the system, the battery will provide power to the load. However, this does not happen instantaneously. Thus, to energize the load during transition from external power to battery power, the system DC/DC converter includes a capacitor that bleeds energy into the load during this period. The longer the period, the larger the capacitor must be. Thus, in the digital switching topology provided in this patent, it is required to have a substantially large capacitor to supply power to the load during the transitional period. Since it is desirable in portable applications to reduce both component count and component size, it would be undesirable to utilize a digital switching topology for these applications.
Similarly, U.S. Pat. No. 5,903,137 issued to Freiman et al. provides a battery pack for use in a portable computing system includes a transistor that is used both for inhibiting charging of the batteries within the system and for limiting the voltage across the batteries in the battery pack. Rather than having two separate devices to inhibit and regulate, this single transistor performs the functions of both, thus reducing component count in a battery pack.
In U.S. Pat. No. 5,994,875, issued to Lee, a battery charging apparatus is provided for use with batteries that require charging in a constant current mode and/or constant voltage mode. The charging apparatus includes a constant current charging control circuit converting the charging current supplied with the battery into a voltage signal and applying the voltage signal to a feedback input terminal of a switching regulator in response to a charging speed control signal F_Q, and a constant voltage charging control circuit providing a control signal to the feedback input terminal for controlling constant voltage charging if the battery voltage level has reached a preset voltage level, whereby a constant voltage charging is possible during the charging operation in response to a charging mode selection signal CHG_MOD. A microcomputer produces the charging mode selection signal CHG_MOD when it is detected the charging voltage of the battery in the constant current mode and the detected voltage reached to a preset level in order to convert the charging mode into the constant voltage mode. Further, a charging speed control signal F_Q is produced to enable the switching regulator to perform quick charging operation. With this arrangement, the constant voltage (CV) charging mode can be performed when the battery is in the preset condition, regardless of type of batteries. In addition, by provision of a protection circuit, possible damage of the CV charging control circuit due to the excessive static or surge is effectively prevented. Other charging topologies can be found in U.S. Pat. Nos. 5,694,025 and 5,969,436.
However, each of these charging circuits require complex components to implement, and are thus costly to manufacture. Indeed, increased pressure exists to provider smaller and smaller components for portable devices, since overall board space and packaging requirements is at a premium. Moreover, none of these references control battery discharge by comparing the discharge current to a preset (programmable) threshold current.
In a single battery system, the prior art examples described above cannot instantaneously control the charge/discharge switch to prevent an overcurrent condition on the battery if the external power source is reapplied to the system. For example, external power removed, the voltage at the battery and the voltage at the common source are equal (or very nearly equal). In each of the aforementioned references, as described above, switch 26 is conducting to permit the battery to discharge. If an AC/DC adapter is then applied to the system, these voltages are still equal, but the switch is still open, which can generate damaging currents into the battery.
Likewise, in a multiple battery system, a cross conduction between batteries can occur if the switches associated with two (or more batteries) are permitted to simultaneously conduct. For example, assume a two battery system where the power source is removed. Referring again to the '137 patent, which discloses a charger topology for multiple batteries. At the time the adapter is removed, all batteries will attempt to supply power to the system. Since all battery currents are greater than power source current, thus the switches 26 will be conducting. However, if one battery has a greater potential than another battery, but both are conducting through the switch, there will be a cross conductance between batteries.
Thus, there is a need to provide a relatively simple circuit topology that prevents these situations, yet satisfies both economy of scale and space requirements.